Phytoplankton abundance and structural parameters of the critically endangered protected area Vaya Lake (Bulgaria)

Vaya (Ramsar site, protected area and Natura 2000 site) is the biggest natural lake in Bulgaria and the shallowest Black Sea coastal lake, which during the last decades has undergone significant changes and was included as critically endangered in the Red List of Bulgarian Wetlands. Our studies were conducted during the summer and autumn months of three years – 2004–2006. The paper presents results on the phytoplankton abundance (numbers, biomass and carbon content) in combination with the indices of species diversity, evenness and dominance. Phytoplankton abundance was extremely high (average values of 1135 × 106 cells/L for the quantity and of 46 mg/L for the biomass) and increased in the end of the studied period (years 2005–2006), when decrease of species diversity and increase of the dominance index values were detected. The carbon content of the phytoplankton was at an average value of 9.7 mg/L and also increased from 2004 to 2006. Cyanoprokaryota dominated in the formation of the total carbon content of the phytoplankton, in its numbers (88%–97.8%), and in the biomass (62%–87.9%). All data on phytoplankton abundance and structural parameters in Vaya confirm the hypertrophic status of the lake and reflect the general negative trend in its development.


Introduction
Vaya ( D Burgas Lake) is the biggest natural lake in Bulgaria and the shallowest Black Sea coastal lake. Vaya is a part of the large wetland complex of several Burgas lakes (including Atanasovsko Lake and Mandra Dam), located on the 'Via Pontica' ornithological migratory route and is important for the conservation of rare and endangered species of national, European and global significance. The lake is a Ramsar site, a protected area in the Bulgarian legislation and the national ecological network Natura 2000. During the last decades the aquatic ecosystem of Vaya has undergone significant changes due to different anthropogenic factors. The main among them are the disturbance of the water balance and the introduction of biogenic elements in the lake, combined with decreased halinity, which have a negative effect on the hydrochemical composition of the water and its flora and fauna. Therefore it has been included as critically endangered in the Red List of Bulgarian Wetlands. [1,2] Complex study of the lake hydrobiology, carried out in 2004À2006 [3] forerunned in October 2003 by hydrochemical studies, [4] include phytoplankton investigation, a part of which, concerning its taxonomic structure, is published elsewhere. [5] In the present study the quantitative characteristics of the Vaya phytoplankton (numbers, biomass and carbon content) and its key structural indices (of species diversity, evenness and dominance) in the changed conditions of the lake due to the anthropogenic impact, summarized in Michev and Stoyneva [2], are presented. A comparison of our results with those from other phytoplankton investigations [5À9] is made.

Materials and methods
The study with the focus on the phytoplankton was conducted during the summer and autumn months of three consecutive years À 2004, 2005 and 2006, as a part of a more complete hydrobiological investigation of the lake physico-chemistry and the invertebrate fauna of Vaya Lake in 14 sites of the lake. [3] In this paper the results from processing of samples from eight selected sites are presented (sample points 1À5, 8, 9 and 11) À Figure 1. These eight sites were chosen also for the evaluation of the species composition of the phytoplankton [5] À the basis of the trophic webs À as most representative in reflecting the anthropogenic impact on the lake and are briefly described in the following.
The sites were visited by a speedboat and detected by means of GPS 'GARMIN'. The following physico-chemical parameters were studied: water temperature (T, C), pH, conductivity (EC, mS/cm), total alkalinity (Am, mE/L), free alkalinity (Ap, mE/L), total acidity (Kp, mE/L), dissolved oxygen (O 2 , mg/L) and oxygen saturation (sat%). [3,4] Since they serve as a background of the studied lake description, main results from them are mentioned here. Recently the lake has a b-mixooligohaline regime. The 1 litre samples were collected in glass bottles and fixed by 2%À4% formalin with further sedimentation in laboratory conditions. Forty-eight samples collected in August and October, 2004, 2005and 2006 were processed in a standard way using Thoma blood-counting chamber on Carl Zeiss Jena Amplival microscope (magnification up to 500£) with the cell as a main counting unit. For biomass calculation in accordance with the cell shape geometrical formulae were used. [10,11] The standard structural parameters of the phytoplankton community À species diversity of ShannonÀWeaver (H), evenness of Pielou (E) and dominance of Simpson (c) À were estimated. [12,13] Results and discussion The total phytoplankton numbers during the investigation period varied between 419 £ 10 6 cells/L and 2071.5 £ In comparison to the detected abundance more than 30 years ago, [6,7] the phytoplankton numbers have risen 147 times for the summer period and up to 300 times for the autumn period ( Figure 6). Similar estimations for the summer period were provided for the lake phytoplankton numbers by Pavlova et al. [9,14] (Figure 7). Similar results were provided for the lake by Stoyneva [8], who pointed out 74.5 mg/L as the summer maximum values for 1995À2003 period, and by Pavlova et al. [9,14] for August 2004.
The biggest part of the total biomass always belonged to Cyanoprokaryota (Figure 8   Our data for the mean biomass value (46 mg/L) during the studied period allowed us to classify Vaya Lake as a hypereutrophic basin according to the open-boundary OECD system as it is compiled in Michev & Stoyneva. [2] They are strongly supported by the cyanoprokaryote dominance, which is generally known to attend the eutrophication events. [15] The strong phytoplankton development, leading to visible water blooms is supported by the high pH values (pH > 10), detected during the summer months, especially in sites 2 and 11 (see the text below). [3] The The results for the phytoplankton numbers, biomass and carbon content, which point the highest abundance and carbon values, as well as the highest contribution of indicative groups of cyanoprokaryotes and euglenophytes in sites 1, 2 and 11, clearly reflect the anthropogenic impact with organic pollution mainly from Dolno and Gorno Ezerovo districts. This is also supported by the data on the phytoplankton species composition, in which Cyanoprokaryota is second by species richness group after Chlorophyta during the same period [5] and eight years before. [8] Additional proof comes from the hydrochemical situation and the low oxygen content in site 1, in particular. There, during the summer, oxygen is almost lacking, leading to anaerobic conditions and appearance of sulphate reduction. In the same site, in winter and autumn, the oxygen content is the lowest for the lake. The results on conductivity also show the highest conductivity values in site 1 during the summer of 2004, related to the anaerobic processes and increase of hydrocarbonate and hydrosulphide ion concentration. [3] The dynamics of the biodiversity index, the indices of dominance and evenness are shown in Figure 11.   The strong decrease of species biodiversity index along with the increased coefficient of dominance and strong cyanoprokaryote dominance generally is known to accompany the change of the trophic level of different natural water basins with achievement of the eutrophic to hypertrophic status. For some work along these lines, see [16À20]. The same was proved for northern Bulgarian coastal lakes. [21] This statement corresponds fully with the results from our recent work, which demonstrate a decrease of biodiversity in the year 2006 with an increase of the coefficient of dominance ( Figure 11).

Conclusions
The data obtained on phytoplankton abundance (numbers, biomass and carbon content) with constant domination of Cyanoprokaryota clearly show that during the studied period Vaya Lake was a hypereutrophic basin. A trend of increase in the phytoplankton abundance has been established in comparison with previous phytoplankton studies, carried out around 30 years ago, as well as within the studied period from 2004 to 2006 [6À9,14] and is generally supported by the obtained hydrochemical results. [3] In this period, the values of the species diversity index and evenness decreased, while the values of the coefficient of dominance increased. The data collected reflect the total  unfavourable direction of the wetland development and confirmed the site being declared earlier as a critically endangered one. [1,2] The proof of the extremely rapid increase of the abundance of the lake's primary producers could serve as an 'alarm' for starting the manangement activities for its restoration and conservation.